text:
Can I use cellular data to receive in-flight weather on my iPad?
I would like to use an iPad for preflight planning. Can I also use the iPad to check weather while I'm en route?

The problem with cell networks is that you must establish contact with a cell and be handed over from cell to cell. If you fly at M .82 you don't have time to complete these operations before leaving the closest cell. This just generate useless control traffic, and the connection is never really established. If there are many devices doing that, the network is perturbated. This is why FCC (in the US) forbid "cell phones" operation in flight, to protect the networks.

Can you? Yes, but service might be slow or spotty.
May you do so legally? That probably depends. The FAA doesn't specifically prohibit the use of any electronics in GA aircraft; they only state that under 14 CFR §91.21, those aboard an IFR aircraft can't operate electronic devices unless they are

Portable voice recorders;
Hearing aids;
Heart pacemakers;
Electric shavers; or
Any other portable electronic device that the operator of the aircraft has determined will not cause interference with the navigation or communication system of the aircraft on which it is to be used. [emphasis mine]

It's possibly questionable that you as the pilot are an authority on whether or not your devices will cause interference.
The FCC, however, has banned the use of cellular phones in flight per 47 CFR §22.925:

Cellular telephones installed in or carried aboard airplanes, balloons or any other type of aircraft must not be operated while such aircraft are airborne (not touching the ground). When any aircraft leaves the ground, all cellular telephones on board that aircraft must be turned off.

Whether or not that can be interpreted to apply to an iPad's 3G connection is pretty iffy.

To illustrate how old that regulation is, the next sentence talks about each cellular telephone installed in any aircraft (it's literally from the "analog cell phones that you install in a car because it weighs 10 pounds who's going to carry that?" days)

@voretaq7, so you're saying I can't use my old bag phone?

In the beginning of this answer you say that "it depends" but later you say that the FCC has banned the use of cell phones.  That seems pretty clear to me (and doesn't really depend).

It depends because the iPad is disputably a "cell phone" based on the definition the FCC is using.

@Lnafziger in this case the iPad is clearly not a telephone (it can't make voice calls over its cellular connection), so the strict letter of the law doesn't apply to it, but my understanding from dealing with device certification is that the FCC has generally held that regulations that apply to cellular telephones apply to anything with a cellular radio. So the iPad falls into a gray area - Isn't legal ambiguity fun? :)

@voretaq7 - Skype works on an iPad :-)

I love the exception they have made for electric shavers. +1

@DeltaLima Yes, I've always found that one puzzling, too. First, that seems like a very random thing to exclude of all of the electronic devices out there. Second, who shaves on an airplane? Do people really do that?

The FCC regulation Title 47 CFR 22 Subpart H is specific to the "Cellular Radiotelephone Service" as defined by those devices operating on certain frequencies in the 800 MHz bands (see §22.905).  That likely means that entire subpart H is obsolete and inapplicable to modern cellular phones.

Unfortunately, in the US it is not permitted to use cellular service in flight because the FCC does not permit it.
On the bright side, the FCC just released a press release which says:

November 21st, 2013
STATEMENT OF CHAIRMAN TOM WHEELER ON FCC PROPOSAL TO INCREASE  CONSUMER ACCESS TO IN-FLIGHT MOBILE WIRELESS SERVICES
Washington, D.C. – Chairman Tom Wheeler has issued the following
  statement:
“Today, we circulated a proposal to expand consumer access and choice
  for in-flight mobile  broadband. Modern technologies can deliver
  mobile services in the air safely and reliably, and the time is right
  to review our outdated and restrictive rules. I look forward to
  working closely with my colleagues, the FAA, and the airline industry
  on this review of new mobile opportunities for consumers.”
-FCC-

That being said, even if it might work at low altitudes, I would highly recommend against it if you are flying IFR.  I have seen videos of cell phones causing flight instruments to act erratically and they can cause major issues if the airplane isn't specifically certified for it.

Can you link up one of the videos of a cell phone causing instruments to act erratically? I have tried to get a cell phone to interfere with avionics and wasn't able to do so. Not that it's a scientific source, but Myth Busters tried it and also failed to get any erratic behavior.

"Cell phones have recently [2013] been permitted by foreign carriers on international flights through use of pico-cells or miniature cellular base stations certified or approved for use aboard the aircraft.  These pico-cells cease operation when the flight is over U.S. airspace" (source: A Report from the PED ARC to the FAA, §3.4, page 14)

@RyanBurnette I saw that episode a long time ago, and I could have sworn that they did get a major variance on a VOR radio, but only with a certain cell phone frequency which is not common.

I have never had much luck with cellular data in flight.
This might just be a function of the kinds of locations where I fly, but in my experience the connection is never good enough to get reliable weather information. Once in a while you might get an update, but I wouldn't count on it.
A much better option (if you can afford it) is FIS-B, which has always worked reliably for me, especially when there really is weather out there (which is when you really need it to work).

XM weather is a good alternative too.

@Lnafziger I'm not sure what's more expensive - the ADS-B receiver, or the XM subscription for a year? :-)

@voretaq7 Neither am I, but they are both good sources for weather.  :-)

Can you get XM on an iPad?

@Magnetoz There are a bunch of XM receivers that work with the iPad.  See http://www.xmwxweather.com/aviation/ for some details, including a hardware link at the bottom that shows the multitude of vendors that you can use.

The weather information is not ADS-B but FIS-B, (Flight Information Service - Broadcast).

@DeltaLima - FIS-B service transmitted on the ADS-B UAT network.  Corrected my answer, thanks!

The cellular antenna radiation pattern is very directional and does not include much upward transmission, so being a significant distance above the towers means you won't get much signal. Also, if flying at a low enough altitude to receive some signal from the towers, at the speeds a large airplane flies you may hand off between towers faster than the devices can handle and your data throughput will be zero because it will spend all its time negotiating new connections with tower after tower.

Yes....ish.
In the US up until recently the answer was an emphatic (FCC-mandated) NO, but the FCC is considering relaxing those rules, and if those rule changes go through you will be legally able to use your iPad's cellular data connection in flight (provided the pilot in command deems it's not a hazard to the flight - so remember to tell yourself it's OK to use the iPad).

Pretty much every CFI I've flown with has ignored this rule, and I'll admit to occasionally checking weather on my phone in flight too. It sometimes works, but in my experience only at relatively low altitudes (I typically lose my cellular signal somewhere between 2500 and 3500 feet AGL. There are long, boring technical reasons for this but basically the way the cell site antennas are designed the signal covers the ground below where cell site is - there's not much signal aimed at the sky above it).
Since it only generally works on climb-out (when my pre-flight weather briefing is still "fresh") or as I'm descending toward my destination (when I'm typically doing other things like talking to controllers or trying to get the field's weather on the radio) I've not found it to be particularly useful to use cellular data to get in-flight weather / information updates.
As Steve said, ADS-B equipment is a much better (albeit more expensive) option.

Of note the FCC is generally talking about permitting use of the cellular radio above 10,000 feet (i.e. "In an aircraft equipped with a microcell") - I'm not sure where they stand on low-altitude use of the cell phone. Part of the reasons for that restriction is the amount of frequency clutter airborne cell phones generate looking for signal and pinging towers.

Also note that there's a maximum speed to allow handover between cell towers before moving out of range, which for GSM (IIRC, was a while back) is around 120-130 mph for ground vehicles. Higher up you should be able to go faster though (speed of divergence is lower). For inherently interruptible traffic like data (as opposed to voice calls) the handover issue shouldn't be a big problem though as you'll just reconnect in the next cell, if there's enough time. :) Other technologies have other specs of course, I know they've tested LTE data using jets in Sweden, so who knows.


text:
Is it legal to takeoff in a crosswind that exceeds the design limit of my aircraft?
Suppose the max crosswind rated for my C-182 is 15kts, but the wind is directly across and gusting to 20kts.  Is it legal for me to go? I've heard plenty hangar flying stories of many an operation that exceeds the design specifications of certain planes that end successfully.

I believe that you should be careful when asking questions such as "is it legal" and answers should only be accepted from people qualified in the relevant area(s) of law. It is legal for me to drive at 100KPH on some roads in my country. But if I did so when the road is icy and I crashed into someone else, then I would not be driving legally and would be guilty of an offence of dangerous driving. Taking off in a crosswind which exceeds the demonstrated crosswind limit and leads to an accident, might make you legally culpable of negligence, but I'm no lawyer, so don't take my word for it!

Some (most?) insurance policies will impose wind restrictions (overall wind and crosswind). These numbers could be higher or lower than the "demonstrated" crosswind mentioned in the POH.

Also check: FAR §23.233   Directional stability and control. (a) A 90 degree cross-component of wind velocity, demonstrated to be safe for taxiing, takeoff, and landing must be established and must be not less than 0.2 VSO.

The 15kt crosswind for the 182 is not a design limit at all. If it were a design limit, it would be specifically stated in the POH. To be honest, I think there's no such thing as a design limit for crosswind. It all comes down to how much rudder authority there's left at a specific crosswind strength. And even that depends on many factors like approach speed, flap settings, weight, CG, etc.
This is what the POH for the C182T says:

Demonstrated Crosswind Velocity is the velocity of the crosswind
  component for which adequate control of the airplane during takeoff
  and landing was actually demonstrated during certification tests. The
  value shown is not considered to be limiting.

and:

The maximum allowable crosswind velocity is dependent upon pilot
  capability as well as airplane limitations. Operation in direct
  crosswinds of 15 knots has been demonstrated

So it is perfectly legal to land and takeoff with a stronger crosswind component. It's up to the pilot to determine if it's safe to do so or not. It all depends what the pilot's capabilities are.

Some airplanes do have a crosswind limit, especially for abnormal procedures.  The rest of your answer about "demonstrated crosswind" is spot on though.

NO
Not if it is actually a design limit.  In the case of the 182 it probably isn't actually a limit though.
With regard to crosswind information provided by the manufacturer, there are two different types.  For example, here are excerpts from the Falcon 50 AFM (which I happen to have handy):
Demonstrated Crosswind Component

Demonstrated Crosswind
Satisfactory controllability during take-off and landing has been
  demonstrated with 90 degrees crosswind component up to 30 kt.

A demonstrated crosswind component is just that.  Demonstrated.  It is the highest amount of crosswind that was encountered during the certification process and is put into the AFM as a guide.  It is not a limiting factor though.  This is what the Falcon 50 has in its POH.
Operational Limits

Operational Limits

Crosswind

Inoperative engine on lee side: 23 kt.
Inoperative engine on weather side: 5 kt.
Center inoperative engine: 23 kt.

In this case, we have an actual crosswind limitation which can not be legally exceeded.  (Note that in this case this is for a one engine inoperative ferry.)
Regulatory Definition
In the case of the Falcon, it is certified under 14 CFR Part 25.  The requirements are similar (but not the same) for Part 23 airplanes.  In particular, we need to look at:

§25.237 - Wind velocities.
(a) For land planes and amphibians, the following applies:
(1) A 90-degree cross component of wind velocity, demonstrated to be
  safe for takeoff and landing, must be established for dry runways and
  must be at least 20 knots or 0.2 VSR0, whichever is greater, except
  that it need not exceed 25 knots.
(2) The crosswind component for takeoff established without ice
  accretions is valid in icing conditions.
(3) The landing crosswind component must be established for:
(i) Non-icing conditions, and
(ii) Icing conditions with the landing ice accretion defined in
  appendix C.
(b) For seaplanes and amphibians, the following applies:
(1) A 90-degree cross component of wind velocity, up to which takeoff
  and landing is safe under all water conditions that may reasonably be
  expected in normal operation, must be established and must be at least
  20 knots or 0.2 VSR0, whichever is greater, except that it need not
  exceed 25 knots.
(2) A wind velocity, for which taxiing is safe in any direction under
  all water conditions that may reasonably be expected in normal
  operation, must be established and must be at least 20 knots or 0.2
  VSR0, whichever is greater, except that it need not exceed 25 knots.

This basically says that they need to test it up to 25 knots.  They can choose to test it higher if they want to, but it isn't required.  If they find that there is a speed less than 25 knots where the airplane isn't "safe", then they need to impose an actual operating limitation.  Otherwise, they just publish the value that it was tested to for reference and it is not a limiting value.

The controlling regulation for Part 91 flights is:

91.9 — Civil aircraft flight manual, marking, and placard requirements.
(a) Except as provided in paragraph (d) of this section, no person may
  operate a civil aircraft without complying with the operating
  limitations specified in the approved Airplane or Rotorcraft Flight
  Manual, markings, and placards, or as otherwise prescribed by the
  certificating authority of the country of registry.

the word "limitation" in this section refers specifically to SECTION 4 LIMITATIONS in the POH.
In the 182 you mentioned, the demonstrated crosswind number comes from SECTION 1 INTRODUCTION.
So you are not prohibited from taking off with a 16kt cross-wind.
If you have an accident with a 16kt crosswind where the crosswind is a factor in the accident, you will still be subject to 91.13:

Sec. 91.13 — Careless or reckless operation.
(a) Aircraft operations for the purpose of air navigation. No person
  may operate an aircraft in a careless or reckless manner so as to
  endanger the life or property of another.
(b) Aircraft operations other than for the purpose of air navigation.
  No person may operate an aircraft, other than for the purpose of air
  navigation, on any part of the surface of an airport used by aircraft
  for air commerce (including areas used by those aircraft for receiving
  or discharging persons or cargo), in a careless or reckless manner so
  as to endanger the life or property of another.

Is it legal?  Most likely.  Is it smart?  Maybe not.  If you fly at an airfield that commonly has crosswinds greater than the MDCWC then you may not have much choice - if you decide to fly.
It is important to remember though that the FAA, NTSB,etc. may not be the only folks interested in this.  Your insurance company may have their own ideas and they are not likely in favor of takeoffs and landings in excess of MDCWC.  This may be a means for the insurance company to deny coverage for any damage resulting.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Do Grumman F-14s, Panavia Tornados, and other swing-wing airplanes count as "fixed-wing aircraft?"
I'm just wondering because the wing isn't fixed, but they aren't rotary-wings either.

Yes - they are still considered fixed wing aircraft. Just because they have variable-sweep sections doesn't change what type of aircraft they are; their aerodynamics, behavior, and performance characteristics are still squarely in the 'fixed wing' category.

Sweeping the wings is really just another way to change it's shape, which is done all the time. Most airplanes have flaps, many have leading edge devices.  These change the shape of the wing to optimize it for different flight modes.
On the gone but not forgotten F-111 everything about the wing changed, sweep, flaps, leading edge slats.
But the Aardvark was a 'fixed wing' airplane.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
When will robots take over pilots?
So, I am considering a career being a pilot. However, the course is real expensive, and I do not want to invest so much money if in a couple of years robots will replace pilots.
So, anyone have any idea when robots will take over?

Robots will not replace pilots in the next couple of years, and possibly not ever for passenger flights.

When they pry the stick out of my cold, dead hand, that's when!

Nobody knows for sure, but I can give you some facts which may help your decision and conclusions:
Technology

Commercial* aviation is one of, if not the, most safety centred industry on the planet. Critical failures and excess risk are simply not tolerated - this means certification is long, expensive and exhaustive.
Because of the above, it can take a long long time for new technology, no matter how small, to filter through into regular use. Changes tend to be evolutionary rather than revolutionary.
Again, to add some context to the above, there is not a single commercial* airliner that is even close to being equipped for autonomous flight. None of them can automatically take off, autoland is extremely limited (Both by conditions and equipment) and they often have functions that simply can't be controlled by the computer yet.

*I'm starring commercial because I'm not talking about military technology or retrofitting random equipment. I'm talking fully developed, certified, approved and proven technology in the commercial aviation space. Flying a drone into a battle is a world apart from flying 500 paying passengers to their holiday.
Rollout
At some point, the above will happen - there's little doubt. However:

The average age of an airliner is somewhere around a decade. If the technology appeared, proven and tested tomorrow and everybody snapped it up, you're talking approximately ten years before the average (not all!) aircraft utilises it.
The above point ignores that airlines have aircraft ordered for YEARS into the future, so you can immediately add a few years lead time onto that.

Requirement
This bit is more opinion orientated and, I'll be clear, I'm not saying commercial aviation will never be automated but I'm not convinced that the outright desire to do so exists right now.
You're saving two crew members on what is already an extremely expensive endeavour, and it's not like they won't be replaced with something. Somebody, somewhere, will need to monitor and control the flight. On top of that, you've got all of the safety requirements that we spoke about above - I'm not sure they can all be met remotely and automatically.
Planes are extremely unique compared to other forms of transport - the distances, the speeds and complexity of flight are a world away from a train or a car which, ultimately, can just stop. Planes don't have a "failsafe" option - you either land properly somewhere or you crash.
I also don't think commercial aviation compares well to military drones. Again, they're worlds apart in requirements, risk tolerance and mission

Would you please give examples of functions that can't be controlled by a computer, or handled remotely?

@rclocher3 electronic failures. Imagine the whole electrical system dies. Then automation will never work

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What runway has the steepest climb requirement on take-off?
What runway has the steepest climb-out requirement upon take off?
This would probably be due to the need to avoid some natural obstacle, such as a mountain.

The first one that came to mind is London City Airport (EGLC) which is world famous for its steep approaches, and for which I found these requirements on its departure charts:

OBSTACLE CLEARANCE -
  RWY 09: Maintain minimum 6.76% climb gradient to 275 (258).
  RWY 27: Maintain minimum 7.20% climb gradient to 1102 (1085).

(Where the first number is altitude in feet and the second number is AAL, above aerodrome level)

Is John Wayne/Orange County/Santa Anna (SNA) at all competitive here? (noise abatement)


text:
In an airliner, does the Captain or the First Officer fly the airplane?
If there is such a thing as a 'normal' flight (in a commercial airliner), who flies the plane? Is it usually the Captain, the First Officer, or whomever needs the hours?

At the commercial airline level, there is very little difference between a captain and a first officer, other than the amount of time that they have been at the company (seniority).
Typically, each of the two pilots takes turns flying the airplane.  For instance, if today's trip is from Miami to Charlotte to Chicago to Atlanta to Miami, the captain may fly from Miami to Charlotte, the first officer from Charlotte to Chicago, the captain from Chicago to Atlanta, and the first officer from Atlanta back to Miami.
The duties in the cockpit are divided between the Pilot Flying (PF) and the Pilot Not Flying (PNF)/Pilot Monitoring (PM).  These duty positions are independent of the captain/first officer designation.  The PF is responsible for physically flying the airplane (usually only during takeoff and landing) and for controlling the autopilot.  If the autopilot acts up, the PF immediately takes over.

can one pilot sleep while the other is flying?

@user13107 Not typically, although I believe that some airlines now allow one pilot to take a 15 minute nap at certain times, but these policies are airline specific and I'm not too familiar with them.

@user13107: Long flights have third pilot on board, so they can take turns sleeping while there are always two flying.

@JanHudec True, and some even have four pilots.  However, he asked about "one pilot while the other..." so I assume that he is asking about 2 pilot crews.  :-)

@JanHudec: If there are three pilots on board and each needs a rest period, will one of them have to switch between left and right seat during the flight?

@HenningMakholm: Obviously yes. Simple matter of combinatorics.

@JanHudec: So the third pilot must be trained/current for both positions? I understand that it's intellectually the same you do on either side, but the photos of flight decks I've seen look like several controls are physically mirrored, so muscle memory wouldn't transfer from one seat to the other?

@HenningMakholm: Most controls are not duplicated at all. You operate them with the other hand, but you operate the same controls (on the centre or overhead panel). And flight controls are not mirrored; left is still left and right is still right. Besides, flying aircraft is about being careful, not fast, so muscle memory is generally not what you need or want.

It's worth noting that the PNF/PM is normally in charge of the radios and reading out check-lists. He's not simply sat there doing nothing while the PF does all the work.

As has been said in previous answers, the duties are divided between the PF and PNF, and the Captain and First Officer typically change which they will do each leg. However, company policy may specify changes to this procedure.
For example, the first 747 carrier I worked for specified that approaches and landings to runway 13 at Kai Tek, the old Hong Kong airport, with it's supposed difficulties, were to be conducted by the Captain. They also had a rule that first officers were not allowed to fly instrument approaches below certain minimums until they had accumulated a certain number of hours in the airplane.
The second 747 carrier I worked for had a rule against first officers taxing the airplane, this a result of some taxi problems they had had with first officers at the tiller.
In all instances, after I transitioned to the left seat at both of those carriers, I ignored those rules as I felt they were not only unnecessary but counter-productive. In a few instances I had first officers decline to do these things when offered, but the vast majority were happy to take advantage of the opportunity.

How dangerous to your career would it have been if there were to occur an incident and it was discovered that you overrode the rules? Is it common for the captain to override airliner rules? I'm just curious, not reprimanding!

@dotancohen We're talking about the 1990s (I hit 60 and had to retire in 1999), and both carriers had a strong it's-the-captain's-airplane culture. It was common for captains to do what they wanted, so I doubt that itself would have been a significant problem. These were small, third level carriers (both now out of business) where everyone knew everyone else, and knew what everybody was doing. That said, as I neared retirement I became more and more independent, including refusing to wear a tie my last 3 months. I was really pissed at being forced to retire at 60. Still am actually.

I see Terry, thanks for the elucidation. At least now, 15 years later, you are still helping others to learn and to love to fly.

You're still bitter? Why, you don't sound it at all!

Both pilots will log time for the flight, although their log entries will be slightly different. The captain will log hours as Pilot-in-Command (PIC).
In practice, "George" (the autopilot) will probably actually fly the plane.

But the pilots both still log the time, even though it's actually auto-pilot?

Yes, definitely.

George still has to be directed by someone, and that is who we normally say is "flying" the airplane.


text:
If the pitch of a bypass fan blade in a turbofan is fixed how does it manage to provide thrust over such a large speed range?
A fixed pitch propeller is only good for aircraft that do around 100kts because the angle of attack on the propeller decreases as the forward airspeed of the aircraft increases, limiting the thrust available. Variable pitch propellers overcome this allowing the angle of attack to remain optimal at speed thus allowing faster cruise speeds.
However the pitch angle of the bypass stage of a turbofan is fixed but yet they do not seem limited by the same problem affecting a fixed pitch propeller. Turbofans provide sufficient thrust at take-off (very low forward speed) as well as much higher cruise speeds.
I'm trying unsuccessfully to understand the physics of this or more specifically: If the pitch of the turbofan is set optimally for cruise, why does it not stall at take-off due to a very high angle of attack? Conversely, if the pitch was set for high thrust at take-off (or some lower speed setting), shouldn't the angle of attack reduce as speed increases thereby limiting cruise speed attainable?

Thanks for the responses. From what I understand there are 2 factors at play:
a) The downwash is greater on a turbofan and this changes the local airflow around the blade causing an otherwise high angle of attack at low forward speed to be below stalling angle of attack
b) The inlet design has the effect of reducing the range of air velocity that the blades actually see and in turn reduces the range of angles of attack. For example, at the start of the take-off roll, there is 0 aircraft airspeed but clearly there is a significant local forward airflow inside the duct.

appreciate any follow up posts! Thanks again.

A large part of this is provided by the turbofan inlet design. Above pic is from Aircraft Gas Turbines by C.J.Houtman and was used earlier in this answer: the inlet shape slows the incoming air down at cruise speed, and trades velocity in for pressure.
The variables:

At TakeOff thrust needs to be maximal, the fan blades need to suck all the air in, and the fan does so at maximum RPM.
At cruise thrust is lower, the RPM is lower, and the air inflow is slowed down.

Indeed the variation in thrust requirements and airspeed create quite a difficult set of operational circumstances to tune, care must be taken to prevent stalling of the blade tips, and stalling of the inflowing air-stream around the inlet nose at lower velocities, which is why the inlet is optimised for these lower airspeeds.
Propellers operate at lower airspeeds than turbofans, and operate at constant RPM with variable blade AoA for optimal thrust efficiency. Turbofans would also benefit from variable blade AoA, and efforts have been made to develop them, but the cost/gain ratio seems to be prohibitive for now.

You say that the inlet is designed to slow the airflow down; wouldn't this increase the AoA of the blades, thereby encouraging a stall? But then you say that the inlet is optimised for lower speeds, where a blade stall should be most likely.

@AdityaSharma The local AoA decreases with decreasing inflow speed. Stall is caused by an inflow with too high AoA, and a way of decreasing inflow speed helps avoid creating blade stall.

I respectfully disagree - decreasing the flow speed increases the local AoA. If you look at the figure in Sophit's answer, you can see that 'V' (incoming flow speed) induces an AoA onto the rotor blades, which reduces their effective angle of AoA. If this 'V' component got smaller with the same 'U' component, the angle 'β' will increase, and the angle between U and W will decrease - this angle is the induced AoA. For a given blade pitch, a decrease in induced AoA means an increase in effective AoA

@AdityaSharma Yeah the blade AoA reduces too much with increasing inflow velocity.

Indeed, and that reduction in AoA is what's preventing them from stalling. The intake design is only making it worse by reducing the inflow velocity, thereby increasing the AoA - the intake is not optimised for low speeds, it's optimised for high speeds where increase in blade AoA is actually needed.

Compressor's (or fan or turbine) blades are designed to properly work in a quite ample range of angles of attack, more or less 15° around their design point.
The speed $W$ seen by the blade (and the relevant angle of attack $\alpha$) is given by the sum of two terms:

the speed $U$ due to the rotation of the blades;
and the speed of the incoming airflow $V$.

This can be seen for example in the following plot (slightly modified by me) taken from this report (blades are seen from above and rotate to left; flight direction is upward):

The rotating speed $U$ can be more or less controlled via the throttle while the speed $V$ of the incoming flow is driven toward the blades (of the first compressor's stage) by an extremely important component that lies in front of the engine and that most of the time passes unnoticed: the inlet.
The main job of an inlet is to reduced the speed of the incoming air to some Mach 0.4, whatever high the speed of flight is. The range from Mach 0 to 0.4 is the limit within which the compressor's blades (and especially their tips) work in a proper way.
If the speed of flight is high subsonic (Mach 0.4 to 1), then the inlet simply works as a divergent inlet (diffuser): its section gets bigger and bigger, the pressure gets higher and higher and the speed gets lower and lower (the compressor thanks Bernoulli).
Finally, if the speed of flight is supersonic, in front of the inlet one or more shockwaves form, across which the speed reduces to high subsonic and afterward a divergent portion of the inlet finishes the work reducing again the airspeed till Mach 0.4.
This is how inlets of a modern jetliner, the Concorde or the Blackbird work(ed) to supply their compressors with a relative slow and stable airflow.

Thanks Sophit. Would it be correct to assume that the enclosed nature of fan blades (preventing spill over the tips) as well as the overlap (or very small gap) between blades is the reason they have a higher stalling angle of attack compared to props? It is harder for air to separate from the upper surface of a blade when there is another blade just nearby to help direct airflow? Thanks again, this is all starting to make sense.

@Ian The exact opposite of that would be correct to assume. Without the duct, there will be blade-tip vortices which will actually reduce the effective AoA of the blades, and that will be beneficial at preventing a stall. So the duct is not helping with blade stall prevention at all - it's doing the exact opposite. However, the closely positioned blades are indeed helping in preventing the stall (by inducing a downwash onto the consecutive blades).

@Ian: the ratio between the distance $s$ between two blades and their chord $c$ is called solidity $\sigma = s/c$. It does has an impact on the efficiency of the rotor and normally has an optimal value slightly bigger than 1.

@Ian: if we analyze the throat between two blades, we have an area of high pressure under one blade and an area of low pressure over the other one. Since in general air moves away from high pressure and toward low pressure, passing through the throat air is "pushed" against the upper surface of the blade and that helps against stall. Obviously there are limits to this effect: if the throat is too large this effect becomes negligible and if the throat is too small the airflow get clogged, that's why the optimal value of 1.

all good. for anyone familiar with sailing, this reminds me of the "slot" between the front sail (jib) and the back sail (main). The slot plays a huge role in optimising the performance of both sails.

One solution I haven’t seen addressed in the existing answers is compressor bleed valves.  The speed with which the air passes over the compressor blades is a function of the pressure differences ahead of, and behind, the blades.  The whole point of multi-stage compressors is for the pressure inside the engine to keep increasing as the air flows from inlet to combustion chamber.
At low airspeeds, and increasing pressures as the engine RPM increases, this means pressure is building up behind the blades faster than the plane is accelerating to create increased inlet pressure.  This is what typically causes compressor stall…  pressure increasing behind the blade reduces the speed of the flow over the blades.
Multi-stage engines will often have a bleed valve in the rear compressor stages that opens to let some air escape until airspeed increases.  This alleviates the pressure building up behind the compressor blades, allowing faster flow, reducing blade AOA, alleviating stall.
Thus the fixed blade angle can be optimized for cruise performance and the engine manages effective AOA by managing pressure gradients inside the compressor.  The bleed valves are pneumatic and operate autonomously with no crew intervention.

Good point, I addressed it en passant in my answer with the picture of the Concorde's engine but it's also important at takeoff.

The AoA of the fan blades varies throughout the speed range, very similar in principle to a fixed pitch propeller. However, the change in AoA also depends on the propulsive efficiency of the engine.
This variation in AoA is determined by the change in "down" wash at the fan blades (think of the fan blade like a wing). This downwash is for to two reasons: forward speed (TAS), and the inherent downwash (jet-wash) due to the production of lift (thrust) itself. On a turboprop engine with high propulsive efficiency, this latter downwash is low, but is relatively high on turbofans (and is even higher on turbojets). The net downwash as experienced by the fan blades is a sum of the two.

When a turbofan is operating on ground at zero TAS, the effective TAS at the fan blades is quite high. This is due to the aforementioned downwash at the fan blades due to the production of thrust itself. This downwash reduces the effective AoA of the blades enough to prevent it from stalling. As aircraft TAS increases, the net downwash increases, causing the effective AoA of the blades to decrease.
However, on a turboprop with a much higher propulsive efficiency, the initial downwash (due to thrust production) is relatively low, and it's influence on the effective AoA is much less. When the aircraft TAS increases through the same speed range as in the turbofan case, the percentage increase in the net downwash is much higher, and so is the change in AoA. The only way to maintain a reasonable AoA is through the use of variable pitch propellers.
In the turbofan case, the change in AoA as the speed increased was significant, but still low enough - a variable pitch mechanism would be able to provide much less improvement, meaning that it's use is not feasible.
If you find this difficult to accept, think of the extreme case: if the initial downwash was infinite (blade pitch would be 90°), an increase in TAS through any speed range would have no effect on the AoA, and so a variable pitch system will be absolutely useless in this case.


text:
Who can perform preventive maintenance? Only owner or owner/operator?
I've a question regarding who can perform preventive maintenance. I had some instructors telling me that only the registered owner can perform it  according to FAR part43 appendix A (c)(30)(i). But FAR part43.3(g) states a pilot with at least private license owning or operating an aircraft can perform preventive maintenance. Which is correct?

Appendix A (c)(30)(i) specifically refers to primary category aircraft:

The inspection and maintenance tasks prescribed and specifically identified as preventive maintenance in a primary category aircraft type certificate or supplemental type certificate holder's approved special inspection and preventive maintenance program when accomplished on a primary category aircraft provided:

It states that the owner (and only the owner) can perform certain tasks in addition to the tasks listed in the rest of Appendix A (c). The operator can still do any other preventive maintenance that is not part of a special inspection and preventive maintenance program mentioned in this subpart.
Primary category aircraft are fairly unusual. They're basically a precursor to modern light sport aircraft. They are type certificated, but under a relaxed process compared to normal category aircraft. You can read about them in AC 21-37.

Is that so the owner positively knows the maintenance was done (because they did it themself) and to a known quality of proficiency/correctly with the qualified parts and procedures - no future post accident name pointing.

@civitas The additional maintenance that can be done on primary category aircraft requires you to go through a training program. It's somewhat of a moot point- primary category aircraft that have been maintained by the owner are not allowed to be rented so the operator is typically the owner anyway.


text:
What is the maximum speed of all the stages of a geared bypass tubojet engine?
What is the maximum speed of all the stages of a geared bypass tubojet engine before they start losing efficiency.

The maximum rotational speed of turbofan blades follows from compressibility effects, just like those for helicopter rotors and for propellers.
Image source
On most high bypass turbofans the fan tips can get to slightly supersonic speeds. A Rolls Royce engineer at the Singapore Airshow mentioned Mach 1.05 for the fan tips, that is why the blades have swept back tip shapes.

All other stages are designed such that blade tip speed remains subsonic: compressibility effects cause a rise in blade drag before the speed of sound is reached. It then drops again, but:

There is a large region of high drag that needs to be overcome, where torque will be waisted on compressibility for no gain in thrust.
Blade root stress is proportional to rpm squared, and it is best to stay in the subsonic region.

For the blades in all stages of the turbofan, three main design considerations are:

tip speed;
blade mass;
operating temperature.

Image source
Blade rotational speed follows from blade radius and tip speed limit. For the fan, a slow turning long blade is best, since it works just like a rotor or propeller does. The compressor blades become smaller at each stage due to the smaller air volume to be compressed, the turbine blades reverse this trend. Rotational speed of the compressor and turbine stages is chosen such that tip speeds stay below the compressibility drag, as confirmed verbally by a Rolls Royce engineer.

Benefit of the gearing can be seen from the shape of the engine: the low pressure turbine blades have a smaller radius than the fan blades, an un-geared engine would turn the LP turbine too slow. With a geared turbofan, the fan tip speed will stay at around the same tip speed, but the RPM of the LP turbine will be brought to a higher value. Below drag divergence number for the hot exhaust gases.

“All stages are designed such that blade tip speed remains subsonic”: are they? I've seen it mentioned couple of times on this site that they actually go up to M1.5.

Compressor blades are not necessarily designed to be subsonic. Front-stages are likely to have supersonic tips. And a fan of high bypass engines does not work like a rotor at all, the fan is followed by outlet-guide-vanes and a annular nozzle.

@JanHudec Checked at the Singapore Air Show, according to a Rolls Royce engineer the fan blades of the Trent 1000 go slightly supersonic, M = 1.05. All other stages stay sub-sonic "..otherwise the drag would be too high."

A question with respect to efficiency is very difficult to answer.
Here is why: Its not entirely clear which efficiency you mean.
Let's assume you are talking about isentropic efficiency of turbine and compressor. Then the most efficient compressor will tend to turn slow and will have a high number of stages. The most efficient turbine will turn very fast, the stage count will also most likely be increase (no numbers here, this section is to get a general feeling for this).
But its not only efficiency which shapes today's engines, it's also Total Cost of Ownership or Life Cycle Cost. More stages will result in more material cost and also in increase maintenance efforts. The engine might be heavier, which influences the wing structure etc.
This is why in the design-phase of a jet engine the design-teams will make trade-studys to find a pareto-optimal design which satisfies as many requirements to the most possible way.
But low TCO is only one target, another one is to ensure a wide range of operating range. This means the engine is supposed to work with a similar efficiency under a wide range of ambient conditions.
This means in order to answer your question adequately you need to add some numbers, like pressure-ratio and/or thrust and/or weight and/or size and/or cost. These boundary conditions will then allow to answer your question.
A study by Lingen Chen et al. investigates the your question with respect to axial compressor design. But all the results are non-dimensional so in order to get a numerical value to your question, you have to plug in a numerical value first.


text:
How common are cancelled landing attempts?
I recently traveled from Stansted, UK to Skavsta, Sweden with Ryanair, and was informed that depending on the weather (it was very foggy), we might end up landing at Arlanda (~150 km from Skavsta) instead.
The pilot later decided to try landing, went down to what felt like 20–50 meters (I could see the airport as clearly as one can see an airport in thick fog...) and suddenly rose sharply and then leveled out, causing some serious roller-coaster-like ~0g feelings.
The pilot then tried a second time and landed without much trouble.
How often does this occur? Was it dangerous? Is it according to standard procedure when approaching an airport in bad weather conditions? What might have caused the pilot to actually land the second time but not the first?

I never ever had a single canceled landing attempt, and neither did anyone else at the club I was flying at. Of course, the fact we were flying gliders might have had an influence on it :)

It's not really relevant to the question as asked, but still a fun, related fact: whereas in civilian landings, there's a lot of margin for error, in carrier landings, if your tailhook doesn't catch the wire, you could simply fall off the end of the deck.  For that reason, all carrier landings are conducted at full or near-full power, so that a failed "catch" results in the pilot simply taking right back off again.  I've heard that on the first carrier landing day for new pilots, there is often a betting pool for how many go-arounds there will be.

@BMDan, technically, every carrier landing is flown as a go-around.  It's just that some of the go-arounds are aborted when the tailhook catches an arresting wire.

Sometimes its not such a dramatic event - I once was in a 747 that had to do a go around as a single runway airport - because there was a rabbit on the field. How do I know this? I saw it scamper away.

@LightnessRacesinOrbit Thanks for the defense, but I gotta agree with SQB here; Mark nailed it.

@BurhanKhalid Rabbits along runways are quite common. Seeing a rabbit scampering away from a side window in the passenger compartment does not necessarily mean it had been in the path of the aircraft.

It was because the pilot announced it as the plane started going up.

@BurhanKhalid But then, that's how you know it…

I could see the airport as clearly as one can see an airport in thick fog...

If you saw the runway by looking down from your passenger seat as the airplane passed over the airport on the missed approach, you were looking only through an amount of fog approximately equal to the altitude of the aircraft above the ground. For purposes of discussion, let's say that was 50 meters. The pilot, however, is looking through the front windshield trying to see the runway ahead to position himself for the landing. If he was on a standard 3 degree approach slope, that would mean he is having to look through nearly a kilometer (beware of old man doing math in his head) of fog.
Years ago when I flew for a commuter, I often had to make multiple missed approaches at Pullman, WA, USA in snow storms. My rule was that I would try 3 times before diverting. When we diverted we would often get complaints from passengers about not landing because they could see the airport as we passed over it. I could, too, but only through the cockpit side window, and with featureless snow covered rolling hills, there was no way to position for the landing without losing sight of the airport.

What might have caused the pilot to actually land the second time but not the first?

The thickness of fog or how hard it's snowing or raining is constantly changing. Try again and you may be able to see what you couldn't before. If you can't, maybe try a third time and get lucky. Trying more that that would mean eating into your fuel reserve to get to your alternate, and, just as importantly in my view, pilot fatigue becomes a problem. Plus, you're beginning to seriously blow your schedule.

ditto. One frustrating night a shallow fog bank forced 2 go-arounds for lack of visibility. Yet flying above it we could clearly see the runway and all the lights. And frankly we tested "landing environment in sight" to its limit when we finally landed.

Q: Was it dangerous? Is it according to standard procedure when approaching an airport in bad weather conditions?

If the aircraft did not go below the Decision Height, no, it was not dangerous and it was according to standard procedures.
Decision Height is defined as

DECISION ALTITUDE/DECISION HEIGHT [ICAO Annex 6] - A specified altitude or height (A/H) in the precision approach at which a missed approach must be initiated if the required visual reference to continue the approach has not been established.
[...] decision height (DH) is referenced to the threshold elevation.
The required visual reference means that section of the visual aids or of the approach area which should have been in view for sufficient time for the pilot to have made an assessment of the aircraft position and rate of change of position, in relation to the desired flight path.

and

DECISION HEIGHT- With respect to the operation of aircraft, means the height at which a decision must be made during an ILS, MLS, or PAR instrument approach to either continue the approach or to execute a missed approach.
This height is usually 200ft.

This means that when the aircraft arrives at the DH, the Pilot Flying (PF) must have decided whether to land or not. If an abort is initiated, he/she cannot modify the decision (as it would be unsafe)and a new decision has to be made during the new attempt.
If the pilot decided to attempt the landing, but conditions change later during the attempt, the pilot can still decide to abort (as in this case). The decision will be based on the current assestment of remaining available runway, aircraft speed, aircraft attitude and current engine throttle.

Q: What might have caused the pilot to actually land the second time but not the first?

The most likely cause is lack of visibility. In borderline conditions, a plane will normally follow an instrument approach down to decision height. At that height they look up, and if they see the runway clearly they land. If they don't they abort. The first time they didn't see it, and the second time they did.
In the general case, other conditions that might cause a go-around are:

lack of preparation – “rushed” approach
a late runway or approach procedure change
an inadequate approach briefing
challenging prevailing wind velocity
inappropriate energy management
inadequate traffic spacing
unfamiliar approach - maybe a straight in non-precision or circling
inappropriate aircraft configuration
runway surface condition
a predicted late touchdown point
unexpected runway occupancy after clearance to land
degraded aircraft systems status
the effect of fatigue
the effect of commercial and personal pressure (stress)

As for whether is common or not, I have no statistics at hand, but a go-around is definitely not unheard of. In Richard's answer there is a statistic for a specific airport (Heathrow).

The link actually has some statistics: 3% unstabilized, 3% of those are go-arounds, so that's 0.09% go-arounds for that reason. But weather-related go-arounds are not (all) included in that number.

@MSalters I meant the % of approaches resulting in a go-around, irrespective of the reason, weather or not.

I think you omitted what is the most likely cause, given the circumstances, which is visibility.

The normal procedure for a poor visibility landing is that the plane follows instruments down to the Decision height. At that height they look up from their instruments. If they see the runway clearly they land. If they don't they abort. I would expect that was what happened here. It's not about an inadequate briefing.

@DJClayworth oh, totally missed that, thanks. feel free to edit the answer.

"Whatever the decision, he/she cannot change it" -- there are videos that show go-arounds initiated after the main gear contacted the runway (e.g. here) -- does that mean that the decision height was nil, or is it actually possible to change the decision in some cases?

@HenningMakholm it means that after DH the pilot proceeded and touched the ground as per decision then proceeded to go-around. Pilots obviously have leeway to ensure the safety of the aircraft, but that directive is to avoid them to second-guess themselves constantly.

Just to be precise (or pedantic), it's allowable (and indeed unavoidable) to go below DH on the missed approach.  When the decision is made, power is applied, but the airplane will continue to sink some before it transitions to the climb.

"..cannot change it..". Agreed. At decision (a.k.a. "minimums") decide. Either continue for landing or go around, you can't do both. As for the practical issues in the answer, certainly in the USAF if we don't have our ahem together by the final approach fix - before minimums on a precision approach - regulations required a go around.

"... no, it was not dangerous and it was according to standard procedures."  In fact it can be dangerous to go around after decision height as TERPS bases obstacle clearance on the assumption you are at decision height when initiating go-around (yes, there is a fudge factor for the additional, slight decent). P.S. we're talking IFR criteria here.

@radarbob In fact I wrote if it was not below DH then it was not dangerous.

@Federico, yeah, I realize that; I'm just trying to make the point that "minimums" is the point where you know that given a "standard", minimum aircraft climb performance you will clear obstacles through the missed approach. Now we have a reference to define "dangerous". Of course a go-around even after touching down can be quite safe 99.9999% of the time. But if you're near Max GW, engine out, flying into a valley on a foggy night you don't have to bet your life on an uninformed WAG.

I don't have any statistics to tell you how common this is, but I can tell you it's absolutely normal. I did it myself two days ago in a small Cessna. Wikipedia says "Go-arounds occur with an average rate of 1–3 per 1000 approaches" but doesn't cite a reference.
The term for what you described is a "go-around": the aircraft cannot land safely for one of several reasons so the pilot decides to increase power, climb higher and go around for a second landing attempt.
A go-around can be required for various reasons, e.g.:

The pilot cannot see the runway clearly enough
There is another aircraft, a vehicle or some obstruction on the runway
ATC instructs the pilot to go around
A sudden wind shift makes the aircraft's approach unstable

Based on what you described, it sounds like the aircraft was on an instrument approach, which simply means that the pilot was following a radio or GPS signal to the runway because the fog prevented him from seeing it. The basic idea behind an instrument approach is that you follow the signal down to a pre-defined minimum safe height above ground and at that point if you still can't see the runway and land safely then you have to go around. I guess that your pilot didn't see the runway clearly enough in time and decided to go around, although it could have been for another reason.
The minimum safe height depends on several factors: the aircraft, the equipment it has, the pilot's training and possibly the airline's own policy on approaches. If the weather is so bad that there is little chance of landing then the pilot can try again as he did in your experience or he can divert to a different airport. Again, there are both legal and company policy considerations on when and where to divert.

How common? Heathrow published (PDF) 0.24% for 2010.
As a passenger I have, just, beaten this statistic with one go-around (at LHR, incoming from Sao Paulo, Brasil) in a total of about 200 lifetime arrivals worldwide.

Yeah, I've also had exactly one go-around (wake turbulence, at MAN) in, I guess, ~100 landings. But, with the average rate of go-arounds being probably less than the average number of landings people have experienced, most people will have experienced either no go-arounds or an above-average number of them.

Averages of small integer quantities will do that to you. Similarly, the vast majority of people have an above-average number of legs.

@Steve Jessop Yes, I know that the noise on random quanta is sqrt(n).  No  statistical significance can be given to my personal experience.  I included it for fun. :)

As a passenger, I've had one go-around. As a (student) pilot, I've had a lot of them. - lol - On the one where I was a passenger, it was a Southwest flight where we stayed relatively high (~15,000 ft) much longer than normal (presumably due to the thunderstorm that we were flying around at the time,) then basically tried to dive-bomb Tampa International. They couldn't get the speed down quickly enough to stabilize the approach, so they went around. We got a nice ~10-15 minute scenic flying tour of Tampa Bay and still arrived on time.

Aborting a landing attempt is called a "go around", because the pilots usually choose to turn around and try again.
London Heathrow reported 551 go arounds in 2010, which is less than 2 per day and represented 0.24% of the total arrivals.
The topic of go arounds came to light after the Asiana 214 crash at San Francisco International last year. According to this information based on the first 7 months of 2013, go arounds happened on 1.31% of approaches for foreign pilots, but 0.28% for domestic pilots (which is close to the figures reported at Heathrow).
Information from CASA in Australia says there are over 800 per year there, but no accompanying number of arrivals is provided.
Using separate statistics, for domestic and international traffic, there are 702321 domestic flights and 162207 international flights in 2013. If only half of the international flights were landing (the other half departing), this represents 783425 landings. Assuming 850 go arounds per year, that is 0.11% of arrivals in Australia. Being averaged over the whole country could account for the lower percentage. Some causes for go arounds such as conflicting traffic would be less common at less busy airports.
According to a consultant on this story, go arounds occur in 0.20% to 0.33% of landings, which matches the other statistics.

So the numbers show that go arounds are not that common, but they do happen regularly and exactly how often this happens depends on many factors. It is much safer to stop a landing and try again than to continue with an approach that could compromise the safety of the flight.
If the pilots have not established the proper altitude and speed in the approach, they should automatically go around rather than attempt to land. They are also required to go around if they cannot see the runway for visual reference by a certain point. If they do not go around, there may be an investigation, because this can lead to a landing accident, which is far more dangerous and costly than just going around.
By going around, the pilots are hoping that the weather conditions will change enough for the next approach to be safely possible. Low visibility weather like fog can easily change in a short time, so if the conditions are only slightly worse than what is allowed, it may clear up just enough to allow a safe landing.

Interesting statistics. Also, Asiana 214 is a good example of why you should abort the approach if you haven't stabilized your approach by the time you reach minimums. Of course, it's also a good example of why you should closely monitor your airspeed on final.


text:
Are there generic routes all pilots can fly?
The recent question Is the destination of a commercial flight important for the pilot? stoked my curiosity but the answers were rather explaining that a pilot does not necessarily need to know where he is flying in advance (and general planning, broadly speaking).
I was more interested to know whether a pilot can fly anywhere. To take an analogy: I have a European driving license. With it, I can drive on all public roads in France and whether I got to Bordeaux or Calais does not matter. The road can be more or less complicated but all of them are accessible to me.
I can also drive to Germany. Sure, there re some minor differences in the regulations, but everything is basically the same.
I can also drive in the US. The differences are more pronounced but at least my European driving license allows me (in short) to drive there.
How is this with pilots?

Are there "generic" airports all pilots can land in? I know that a special license is required to land on some (the old airport in Hong Kong for instance) , but does this mean that "any airport is OK, except the ones which require special licenses?)
Same question for routes: are all domestic / international / overseas routes open to all pilots? Or one needs to have a special permit to fly one of these?

I know that each airplane type requires a license (or families of aircrafts) - similarly to my driving license type B for which a family car is OK but a truck is not. My question is on routes and destinations, once you fly something you are allowed to.

In the US (and most areas of the world, I'd imagine), you may not land at a military facility without prior approval, or calling an emergency. However, if you do report an emergency and request to land on a military base, be prepared to be greeted by some grumpy faces and do a lot of explaining. (There's a question about that here somewhere, but I haven't searched for it.)

@FreeMan: And quite possibly the people wearing those grumpy faces will belong to people carrying automatic weapons :-)

@jamesqf most assuredly so!

In short, yes it is a lot like travel on the road. There are treaty agreements that came from the Chicago Convention on International Civil Aviation in the 1940s which allow quite a substantial freedom of international travel and cross recognition of pilot certificates.
In the USA most routes are open to most pilots, the restrictions are based on the conditions and chosen rules of flight. In an emergency the pilot is legally allowed to do anything needed to address the emergency and get the aircraft on the ground without regard for any routes or other rules, as such I will not mention the emergency exceptions for each separate item and situation.(91.3[b] in an in flight emergency requiring immediate action, the pilot may deviate from any rule of this part to the extent required to meet that emergency)
As you said for driving, the rules of driving may change by country but this applies to all drivers equally, it is mostly the same for pilots.The restrictions for pilot certificates mainly relate to the carry of passengers or cargo for hire not the location or route of flight, similar to the divide between private family car and a taxi or delivery service. Exceptions are the very limited hobby flight categories (ultralights, light-sport, and recreational) but this would be similar to a small moped or Vespa which is not allowed on highways marked above 40mph(60kph).
Flying under visual flight rules generally does not strictly adhere to designated routes. The primary designated routes are federal airways(for the space between) and preferred departure and arrival procedures(near the airport), these routes are actually designed to assist instrument flight rules flight planning and the ATC managing said IFR traffic, however following these routes while VFR is allowed. VFR is literally visual, just look out the window and see where you are traveling. Flying under instrument flight rules will require an instrument rating on your pilot certificate, however the instrument rating applies to both private and commercial certificates.
Certain airports are designated as ports of entry and have customs offices, just like old time marine ports and road border crossings; a port of entry is the first landing after crossing the border, though it may be hundreds of miles from the border. A filed flight plan is recommended to reduce border problems but not strictly required in all cases.(see Chicago convention) A port of entry is open to domestic flights just the same as any other public airport.
There are additional restrictions on the choice of route and airport for regularly scheduled commercial operations, as compared to unscheduled air-taxi and private operations. Also the practical physical limits of the airplane and landing surface; length, width, strength of pavement, climb over local obstacles, etc.
We [USA] have airspace classes A, B, C, D, E, G and "special use". A and B are positive controlled meaning that you must negotiate with ATC for clearance to enter. Most special-use are associated with military training or high security areas, though some are simply alerts or caution areas for other potential hazards such as a large flight school in the area.

Class A is everything between 18000 feet above sea level to flight level 650 (about 65000 feet depending on barometric pressure) and class A requires instrument flight rules which will require both filing a flight plan and an instrument rating on the pilot's certificate.
B is used for the busiest airports, every flight in B must talk to ATC and get explicit clearance to enter B, IFR flights get priority. When ATC is very busy VFR flights that are just passing through and not landing within the class B area may be turned away. B generally covers an area up to 30 miles from the primary airport, though it has a complex shelf structure that steps up as you get further from the airport and allows other airspace below and above the shelves.
Classes C and D are also airport tower controlled but they are smaller than B. C is around secondary international and primary regional size airports, and D is for the least busy airports with a control tower.The main difference is that class C requires a radar transponder with altitude encoding and has a 10 mile radius with one shelf, D has a 4 to 6 mile radius no shelf and no transponder requirement. Both require two way radio acknowledgment from ATC but they do not require explicit permission to enter, planes with no radio can be accommodated if arrangement is made with ATC at least one hour prior.(this is to allow for antique aircraft or flights to repair stations) C, D, and E are controlled but not positive-control airspaces, as such IFR traffic is looked after and kept separated by ATC while VFR traffic just minds its own business for the most part, mainly contacting ATC during landing or for departure. Many D airports are so small they close the tower at night and revert to class E or G pilot-controlled airports.(The Boeing factory airports are all public class D) My local air-force and army-airfield bases are both class D and while I usually just cut through (near my preferred airport and I don't want to take the long way) they do allow some non-military landings with prior permission.
Class E is controlled enroute space, it is all the area between B, C, and D where ATC support is available. Low level Federal airways are all in class E, VOR "victor routes" are the most common type and usually are from 1200feet above ground to 18000 feet above sea level and 8 miles wide, though there are a few T routes [GPS or inertial navigation]. These routes are designed for IFR but they are not restricted, any VFR pilot can fly on them or in between them, though IFR will fly at 1000ft altitudes[eg 5000, 6000] and VFR will add 500ft[eg 5500, 6500]. E also covers air above FL650, though all airspace above FL450 is considered point to point and there are no designated routes. Most airspace above 1200ft is now E, some places E starts at 700ft or at the surface near some airports.
G is all the space between where ATC is not available, also known as uncontrolled airspace, mostly found from ground to 1200ft, where ATC does not have radar coverage G can go as high as 14000ft. G has no IFR because there is no ATC, and no designated airways because there is no IFR, the grey middle is a class G airport with an instrument approach, generally the instrument approach requires minimum cloud base/decision height above the G to E boundary. Flight in IMC is allowed in G without an active flight plan, at your own risk.
Special use airspaces are of several types and can be active only select hours (military training fighters or artillery range) or full time; and they can be only an alert to hazards like heavy student training or moderate military operations; "restricted" during certain hours for heavy military use (though you can still radio the controlling agency and ask them for permission to pass); or fully prohibited.(the only prohibited around here is a ballistic missile submarine base, and it only extends to 2500 feet above the surface)

We also have VFR flight plan filing in the USA, but this is voluntary and not permission to fly. A VFR flight plan is only a statement of intended route and estimated time so that a search can be made if you do not contact the service within 30 minutes of the stated arrival time. Flight plans can be changed in flight or canceled and closed by the pilot over the radio without actually completing the trip as filed. IFR flight plans can also be filed, opened, changed, canceled, and converted to VFR by the pilot while in the air.

Excuse the poor formatting and typographic errors, I was short on time. I'm still to busy to edit.

Generally an excellent answer but it's not correct to say that G has no IFR. You note correctly that flight in IMC in G is permitted, but it must be done in accordance with the instrument flight rules (e.g. Minimum IFR Altitude, alternate requirements etc) even though it does not require an ATC clearance.

Airports
In general, there are three main types of airports: public, private, and military. You can visit the same types of locations in your car, and they have somewhat similar restrictions.
Military and private airports will typically require prior permission to land there, and will want to know what business you have going there.
Public airports will depend on the size and usage. The largest airports will require coordination with ATC, and scheduled air carriers will get priority over a small general aviation aircraft or unscheduled arrivals (excepting emergencies). Even at less-busy airports, just as with visiting public areas in your car, there maybe additional restrictions such as times of operation, and you should do your research ahead of time to make sure you will be allowed access when you get there.
Routes
Route access will mostly be determined by the class of airspace they go through. Different types of airspace have different restrictions, and this ties in with the discussion on airports. The busiest airports in the US are surrounded by class B airspace, and ATC has to give you permission before you enter these areas. There are also areas with special restrictions, either for military use or safety/security reasons. The restrictions range from encouraging pilots to use extra caution, to encouraging them to avoid it, to prohibiting any aircraft that don't have permission to be there.
In the US, there are Victor airways, which are aimed towards use by smaller aircraft flying lower than airliners typically do, and designed to avoid restricted areas whenever possible. Most of the airspace in the US under 18,000 feet is class E, which means pilots under instrument flight rules, including airliners, will have to talk to ATC, but pilots under visual flight rules are mostly free to operate on their own.
There are also restrictions on equipment. Victor airways are designed so that pilots can navigate them with a VOR receiver, which most aircraft will have. There are other airways which require more accurate equipment like inertial navigation or GPS to follow. If a pilot flies such a route in airspace where they don't have to talk to ATC, it's up to the pilot to ensure they can safely follow their route.
When moving between countries, there may also be general restrictions about whether your license allows you to fly there all. This will depend on the specific countries involved. For example, see: I have a European EASA license - what do I need to do to be allowed to fly in the US?

Are there "generic" airports all pilots can land in?

In the US, there are some Large airports (8-10 or so), really busy, that require a landing reservation time slot.  Most others, if you can land in the available runway length, you can fly there and land with no restrictions.  I personally try and stay away from places with lots of jet traffic, they create all kinds of turbulence that can upset my little 4-seater.  With 5000+ airports in the US, it's not hard to find someplace that is small airplane friendly.

Some of the most interesting YouTube videos I have ever watched are of people taking their 172 into fields like ORD or LAX. It's a lot of work, even at 1 am.

My chart supplement and far 93.123 only list LaGuardia, Newark, O'Hare, Reagan, and JFK as high density traffic airports requiring reservations. Reservations are not required between 2400 and 0600 local time. Non-scheduled and VFR operations are allowed if they request a reservation from ATC and ATC determines they will not pose undue delay to normally scheduled operations, these unscheduled operations are not counted toward the airport's operations per hour limit/quota.

You need to have up-to-date charts of the airfield you are trying to land at but other than that only the difficult airports require extra certification. Though many busy airports will charge a fee for the privilege of landing there.
Those charts will have all the information needed for a pilot to land at the airport. This includes nearby navigation markers, the approach path(s), decision height and missed approach procedure.
If you stay in VFR then you can use all VFR routes. However if you want to go along a IFR route you need to file a flight plan and have it approved. On departure you would "pick up the clearance" and then fly along the route talking to ATC along the way.
Flying IFR requires that you are certified to fly IFR.

One can't fly VFR, unfiled, along an IFR route while maintaining VFR altitudes (even and odd thousands)?  I'm pretty sure you can.

@CrossRoads With route I include the required altitude ranges, so I would consider an IFR route followed in the VFR airspace below it a VFR route.

Fair enough.  I see the airline-pilot tag, missed that earlier. Are the airlines all filing IFR?  I know when I go cross country to new areas I file IFR, just to make sure ATC doesn't dump me when I get away from a large airport environment.

@CrossRoads Almost all airlines file IFR (save for some small puddle jumpers that don't get above a few thousand feet). IFR is required above 12,000 feet so that pretty much means everybody files IFR.

"go-around procedure" I think you meant the missed approach procedure.

@Ron Beyer: I think (in the US) it's "above 18,000 ft", unless somebody changed the rules without telling me.  There's quite a bit of country around here where the 12,000 ft level is occupied by granite :-)

@jamesqf You are right, I mixed up some numbers in my mind

I can also drive in the US. The differences are more pronounced but at least my European driving license allows me (in short) to drive there.

Your license allows you to drive in the US, but it does not grant you entry.
I wasn't certain if this was the intent of your question, but just because traffic control clears you on an international route doesn't automatically grant you entry into foreign airspace.  It depends on the country.  Some will allow any flight in to a designated entry facility.  Others require valid permits prior to entry.


text:
How could a failed left wing cause a roll to the right?
The crash of Continental Express flight 2574 in 1991 (see also this question) was caused by faulty maintenance to the de-icing boot on the left horizontal stabilizer. This became detached in flight, leading to violent movements of the plane which caused the left wing to fail and the right wingtip to detach.  Ultimately, the horizontal part of the T-tail detached and the plane crashed.
The NTSB report says (page 34, with essentially the same statement repeated on page 37):

The airplane then rolled to the right at a roll rate exceeding
  160 degrees per second. The Safety Board believes that the lift produced by the
  intact right wing produced the extreme roll.

The report also says that the initial failure of the left stabilizer caused a 10–15° roll to the right, which makes sense: the loss of downforce from the stabilizer meant the left side of the plane was generating more net lift than the right. But how would the subsequent failure of the left wing produce a roll to the right? The natural assumption would be that, with the right wing generating lift and the left wing not, the plane would roll to the left.
Possibly relevant is that the left wing failed but didn't detach from the plane: the plane crashed right-way-up with the left wing folded under the fuselage and the inboard portion of the right wing.

The following is how I would interpret the information given in the report - this is to some extent speculation, also because the FDR plots in the report don't seem to cover that part of the incident. I'm very intersted in how other people interpret the data.
The report states:

The airframe remained intact (minus the leading edge), and the load
  factor fluctuated around -2 g, for approximately 6 l/2 seconds. [...] airplane heading moved 20 degrees nose left, and a 15 degree right roll attitude was reached at the end of this period.

The fact that the left leading edge of the stabilizer was lost and that the heading is moving left makes me believe that the aircraft was in some kind of a slip with the right wing further forward in the wind. The negative load factor of -2 g indicates a lot of negative load on both wings - apparently more than the structure tolerated. Both wings "stalled negatively"1 at this point. Now if one wing goes missing (aerodynamically, even if not physically), it can't provide the downforce any more and the total momentum will cause the side of the aircraft where there is still a functioning wing to rotate down.
Because the right wing was probably pointing slightly forward (even more so if the left wing produced a lot of drag while or after failing, which seems likely), a roll to the right would make the aircraft more or less tumble over its own wing, with the wing upper surface pointing increasingly towards the airflow around a 90° right roll position. The resulting drag explains a very fast roll to a ~180° position. Starting from "10 to 15 degrees right wing down"2, that's about 160°.
Short version: Because the wings had negative load and were primarily drag surfaces at this point, everything is reversed. The side of the aircraft with less drag rolled towards the incoming airflow. In a negative angle of attack attitude, that's up in the aircraft coordinate system, causing a roll to the right.

1 3.1: Findings; 7. p. 48 (56 in pdf)
2 p. 20 (28 in pdf)

That makes a lot of sense. Your suspicion that the aircraft was sideslipping with the nose left (so right-wing advanced) is confimed by the report on p36 (second paragraph).

Ah, and the point is that lift is force perpendicular to the direction of airflow. Normally, this is an upward force but, because of the configuration of the aircraft, it was a downward force. The right wing generating downward lift obviously rolls the plane to the right.


text:
What causes aileron and elevator flutter?
This video shows a Hawker jet with the wing fluttering up and down like it's about to break. What can cause flutter like that? Can it actually cause a wing or stabilizer failure? How can flutter be prevented? What should be done if something like this happens?

The description on https://www.youtube.com/watch?v=s3-g9B6Fgjs is not too bad. Usually the only way to stop flutter without breaking anything is to slow/brake down really quickly. I also like https://www.youtube.com/watch?v=RenbFgLZBNA, plus there is the linked PDF.

In the case of that Hawker we definitely seem to be looking at a resonant vibration in the wing (think the Tacoma Narrows Bridge, but flying), likely due to some "incorrect" maintenance done on the plane at some point. The FAA SAIB linked in the video description (NM-14-05) is worth a read, though it doesn't really answer your question.

@voretaq7 - your comment would make a terrific answer.  Flutter is all about resonant vibration, & one of the reasons we have Vne

I guess altitude and winglets are factors. http://www.ainonline.com/aviation-news/ainalerts/2013-05-09/bulletin-advises-fl340-limit-api-wingletted-hawkers

Imbalanced or loose (and, in extreme cases, structurally weak) control surfaces can cause flutter, a type of harmonic motion. Flutter is a very dangerous condition; if it is not stopped, it can cause structural failure and potentially lead to a fatal accident. V-tail Bonanzas had a series of such accidents related to flutter of the butterfly ruddervator tail.
I understand that prevention is a combination of good maintenance and keeping the aircraft below Vne (never exceed speed), or possibly other speeds in some configurations.
The appropriate reaction would be to slow down, first and foremost. Descending is probably also a wise choice.

Poor maintenance can definitely be a cause of aileron flutter in light aircraft (worn aileron rod end bearings that allow enough play for resonant vibrations to set up are a pretty classic example, and part of why we check the ailerons for free play during preflight).

Flutter is essentially an aerodynamic and structural interaction that has gone unstable. At certain speeds and flight conditions, the aero forces interact with the structural dynamics of the aircraft. This aero and structural dynamic instability can be seen in other places other than aircraft as well, the most famous that comes to mind is the one with the bridge rocking back and forth before collapsing